The search  part of the macro  would look like

Dim MyWorkbook As Workbook
Dim MySheet As Worksheet
[FSO Code to open each workbook in turn  here]

MyString = "error"
For Each MySheet In MyWorkbook
x = MySheet.UsedRange.Find(MyString).Activate
If Not x Is Nothing Then
MyFlag = 1
End If
Next

The FSO will find the workbooks and open them. You will not be able to access
the contents of them without opening them in the application. (Unless you use
a text  search facility completely outside VBA).